On my desktop, running Chrome, I can just select full screen from the menu and it displays retool in full screen. this is true for preview, edit and running mode.
On my Android tablet, I can't see the option to go into full screen. I tried Chrome and Brave browser so far.
Im not sure if you use Mac or Windows.
On Mac, in View section you can toggle off toolbar showing in full screen and then Command + Control + F would get you to full screen.
For Windows, and I might be mistaken here, did not use it for a bit, but Alt+Shift+A to hide the toolbar and F11 to get you to full screen.
This is for Chrome btw.
Just to confirm, does it work for non-Retool pages? I don't have an Android tablet to test with, but I was reading this external post and wanted to double check